Persistent pulmonary hypertension of the newborn (PPHN) is a serious clinical problem that affects up to 10% of infants admitted to the Neonatal Intensive Care Unit, and occurs when pulmonary vascular resistance does not decrease normally during the transition to air breathing at birth. We have utilized a fetal lamb model of PPHN to study the vascular changes associated with PPHN, and to determine how these changes might interfere with the normal pulmonary vascular transition. Our preliminary data in PPHN lambs strongly suggest: 1) a central role for reactive oxygen species (ROS) in the pathogenesis of PPHN; 2) that hyperoxia enhances production of ROS by mitochondrial and cytosolic sources, which increase vascular reactivity, and 3) that the PPHN lamb is particularly vulnerable to the effects of hyperoxia, responding with exaggerated increases in ROS and vascular dysfunction. We hypothesize We propose three specific aims to further our knowledge of the pathogenesis and therapeutic opportunities for PPHN: 1. Determine abnormalities of vascular mitochondrial and cytosolic ROS production specific to PPHN. 2. Determine the effect of hyperoxia on ROS production in the PPHN vasculature. 3. Determine the effects of therapeutic strategies targeted at reducing ROS production in specific cellular compartments. An improved understanding of the mechanistic basis for this syndrome will potentially identify novel clinical approaches to managing the vascular dysfunction that characterizes PPHN. Such therapies could dramatically improve immediate and long-term clinical outcomes, while substantially reducing the health care costs associated with this syndrome.